diff --git a/ws.php b/ws.php
index f49ff8e74..398aea2d4 100644
--- a/ws.php
+++ b/ws.php
@@ -823,7 +823,7 @@ function ws_addDefaultMethods( $arr )
       array(
         'image_id' => array('default'=>null, 'flags'=>WS_PARAM_ACCEPT_ARRAY),
         'pwg_token' => array(),
-        'category_id' => array('default'=>null, 'type'=>WS_TYPE_ID),
+        'category_id' => array('type'=>WS_TYPE_ID),
         ),
       'Notifiy Piwigo you have finished to upload a set of photos. It will empty the lounge, if any.',
       $ws_functions_root . 'pwg.images.php',